This invention relates to antipilferage tags or markers. Such tags are applied to articles of commerce in order to protect them from theft at the point of sale premises. Typically, the tag is a magnetic medium which is deactivated when a shop assistant carries out a routine procedure at the time of effecting a sale. Such deactivation prevents detection of the magnetic tag when it (and the article to which it is attached) pass through a detection system, typically in the form of a walk-through framework which emits an alternating magnetic interrogation field. This field is designed to interact with a tag prior to deactivation and, in substantially all known prior systems, to cause a warning signal to be emitted in the event that detection of a non-deactivated tag occurs.